This invention relates to temperature sensing circuits comprising insulated gate field effect transistors (hereinafter termed "IGFETs"), particularly but not exclusively suitable for integration with a power semiconductor device, for example a power IGFET (hereinafter termed "MOSFET"), a power insulated gate bipolar transistor (hereinafter termed "IGBT"), or a power bipolar transistor.
U.S. Pat. No. 5,336,943 (our reference PHB 33762) incorporated herein by reference, discloses such a temperature sensing circuit which comprises first and second IGFETs of the same insulated gate field effect type as each other. These IGFETs are coupled in separate current paths from each other so as to have respective gate-to-source voltage signals between their source and gate electrodes. Each IGFET is a diode-connected transistor having its drain electrode coupled to its gate electrode. The first IGFET is operated deep in its sub-threshold region where the voltage across the device varies with temperature. The second IGFET is operated in an area of its square law region where the voltage across the second IGFET is substantially independent of temperature. A comparator circuit compares the voltages across the first and second IGFETs to provide an output signal indicative of the temperature sensed by the first IGFET. Generally two or more series-connected first IGFETs are required to provide an adequate signal level for the comparator circuit. The second IGFET provides the comparator circuit with a reference level corresponding to a temperature threshold. Other temperature sensing circuits using temperature-sensing IGFETs are also referenced in U.S. Pat. No. 5,336,943. The whole contents of U.S. Pat. No. 5,336,943 are hereby incorporated herein as reference material.
An alternative type of temperature sensing circuit using a temperature-sensing p-n diode is disclosed in published PCT International Application WO97/02592 (U.S. Pat. No. 5,726,481; our reference PHB 33990) and the references cited therein, for example, EP-A-0 414 499. The whole contents of these documents are hereby incorporated herein as reference material. This alternative type of circuit comprises a current path through a temperature-sensing p-n diode means to provide a voltage drop having a negative temperature coefficient. Different circuits are possible in which the p-n diode means is either forward-biased or reverse-biased. Generally, the temperature-dependence of the p-n diode forward characteristic is used for temperature sensing, even though the reverse characteristic is known to have a stronger temperature dependence. Thus, the leakage current of a reverse-biased p-n junction increases exponentially with temperature. In the case of a reverse-biased p-n diode the negative-temperature-coefficient voltage drop is generated across a resistor by this current. In the case of a forward-biased p-n diode the negative-temperature-coefficient voltage drop is generated across the p-n diode itself. The temperature-sensing circuit also includes an amplifier comprising an IGFET, the gate electrode of which is coupled to the p-n diode means to provide at its drain electrode an output signal indicative of the sensed temperature.
Such temperature sensing circuits have proved advantageous for regulating the operation of a power semiconductor device to protect it from overheating. Such circuits are particularly advantageous in the field of socalled "smart-power" devices, both to protect the power device section from overheating and to maintain proper operation of its logic circuitry. In this case, control functions provided by low-voltage logic circuitry are integrated at low cost with a high-voltage power device to control its operation, and an improper logic function may result from an excessive temperature increase. Although these known temperature sensing circuits work well, the present applicant finds that a significant spread in the threshold temperature can occur for the same circuit manufactured at different times, i.e different batches of the same circuit. It seems particularly important to control variations in process parameters associated with the IGFET threshold voltage in order to reduce this spread.
It is an aim of the present invention to provide an alternative type of temperature sensing circuit in which the threshold temperature is less susceptible to variation in the process parameters associated with an IGFET threshold voltage, and to provide also an opportunity to adopt a simple integrated circuit configuration.